All, I dug some more, and pulled the box out of the rack to hook up a VGA card (old box w/o built-in VGA or serial-redirection), and discovered that PCI bus-mastering was disabled for the IDE controller. I enabled that, as I think it's a requirement for DMA operation.
Second, Frank Smith suggested recompiling the kernel with specific support for the IDE controller. I chose not to do that, but it did make me realize that I did not have the PIIX4 driver loaded (piix.o). I'm not sure how I was able to use the disk without that, but once I loaded that module, DMA was enabled auto-magically. I am now able to pull 28 MB/s from this disk in UDMA(33) mode. Thanks for prodding my gray matter, Frank! Tomorrow morning's run should be the final test! Eric
